1. Field of the Invention
The present invention relates to a sensor for the efficient inspection and measurement of objects such as semiconductor wafers upon which dielectric and conducting films are deposited.
2. Description of the Related Art
In the process of manufacturing integrated circuits, films are deposited on semiconductor wafers, etched, and overlaid by additional films to create the integrated circuits. The films may be either conducting metal films or dielectric films. Quality and thickness of a particular film layer is critical to the manufacturing process, and it is of particular interest to measure the thickness of a film layer quickly and accurately. Two types of measurement techniques are commonly used to measure thickness. Dielectric films are transparent and thickness may be measured using optical reflectance. Conducting metal film thickness may be measured using eddy current sensing techniques. Further, if the conducting metal film is thin enough, it is transparent or semi-transparent which allows the thickness of the film also to be measured using optical reflectance. Both techniques are known and used in the industry.
Optical reflectance may be used to determine characteristics such as thickness, index of refraction and index of extinction of transparent and semi-transparent films. Metal films with a thickness greater than a few tens of nanometers tend to be optically opaque to light having wavelengths between 200 and 1100 nanometers. As a result, optical reflectance measurements are ineffective to determine thickness for such films. Eddy current sensing may be used to measure sheet resistance, which is correlative of such electrically conducting film thickness. Thus eddy current sensing generally may be used to measure thick conducting films and optical reflectance sensing generally may be used to measure thin conducting films that are transparent or semi-transparent.
Eddy current sensing techniques do not work for measuring thickness of films which do not conduct electricity sufficiently such as dielectric films. As a result, it is necessary to rely on optical reflectance techniques to measure dielectric film thickness.
The optical reflectance of the conducting metal film can vary with surface oxidation and grain size. After a conducting metal film is deposited on a semiconductor wafer, an annealing step is carried out to stabilize the film, to improve the conductivity of the metal film by enhancing the size of the metal grains, and with the proper reduction agent, to clean oxide from the film. This annealing process changes the grain size of the conducting metal film which changes the optical reflectance characteristics and the sheet resistance. The change in film reflectance after annealing can be correlated to both oxide removal and grain size variations.
Moreover, eddy current sensing alone cannot distinguish whether sheet resistance varies because of film thickness or because of grain size change caused by the annealing process. Therefore, both optical reflectance measurements and eddy current sensing measurements are required to adequately determine thickness of a conducting metal film, and it is advantageous to take both measurements during a single inspection. Optical reflectance measurements can compensate for variations in sheet resistance due to grain size variations thereby allowing eddy current sensing measurements to correlate accurately with film thickness.
Both the optical reflectance and the eddy current sensing methods of inspecting conducting film are well known to skilled practitioners of the art. However, each method has been employed separately and apart from the other. In view of the foregoing, it would be desirable to be able to rely on different types of information derived from a combination of both optical reflectance and eddy current sensing measurements.
One feature of the invention is to provide both optical reflectance and eddy current measurements for determining film thickness. Reflectance measurements are useful to determine grain size, oxidation thickness, and film thickness when the film is transparent or semi-transparent. Both dielectric and conducting films may be inspected using optical reflectance measurements. However, only eddy current sensing techniques are useful in measuring sheet resistance of conducting film which is determinative of film thickness and grain size. Such eddy current measurements are typically made at a distance of a few microns to hundreds of microns from the film and cover an area of 1 to 2 square millimeters.
The inventive design enables both eddy current sensing and optical reflectance measurements to be taken sequentially or simultaneously. When both eddy current and optical reflectance measurements are made by the same sensor, either dielectric or conducting metal films may be inspected without changing the sensor. Thus, manufacturing time is reduced because any time needed to change sensor types to inspect different types of films is eliminated. Further, a film may be inspected while it remains in the process chamber thus eliminating time needed to move a wafer with a deposited film from a process chamber to an inspection chamber. Still further, multiple film layers may be inspected simultaneously provided a transparent dielectric film overlays a conducting metal film.
In accordance with the invention, a two optical fibers (or a bifurcated optic fiber having two channels) and an eddy current inspection coil are provided together. In one embodiment, the fibers may be placed co-axially within the coil. A first optical fiber is connected to either a broadband or a narrowband (laser) light source for illuminating the film. A second optical fiber is connected to a spectrometer or a monochromator which is equipped with a charge coupled device, a photodiode array, or a photodiode with a band pass filter to measure the reflected light intensity returning from the object through the second fiber. Without optical focusing, the area illuminated by the first fiber is about the same area as the area inspected by the eddy current sensor. However, some applications may require a smaller inspection area, and in those cases, a sensor which includes an optical focusing member may be used.
In another embodiment, the first and second optic fibers are mounted external to the eddy current sensor but are focused on the area to be inspected by the eddy current sensor. Connections to the light source and detector remain the same as in the first embodiment.
In still a further embodiment, an array of eddy currentxe2x80x94optical reflectance (eddy-optic) sensors may be used to measure the uniformity of a film""s properties, or a single such sensor may be used to measure the properties of the film at various locations by effecting relative movement between the sensor and the film, by moving either the sensor, or the film, or both.